RESPONSES TO ODORANTS 63 



series of amyl acetate are shown in Fig. 16. The question is frequently 

 asked whether the olfactory receptors show adaptation, or is adaptation of 

 olfactory sensation largely a central nervous system phenomenon? The 

 answer is that phasic responses occurring with free respiration are main- 

 tained indefinitely for low levels of response, but for more intense stimula- 

 tion the responses decrease to a level which is maintained thereafter. Such 

 depressed levels of response can approach complete absence of activity 

 for some odorants. e.g. chloroform. However, for continuous stimulation 

 achieved with unidirectional flow of odorous medium through the nose, 

 the tonic response is always less than the phasic response. One presupposes 

 eHcitation of the phasic response, for which it is necessary to attain the 

 stimulus concentration in a sufficiently short time. Figure 16 shows also 

 that after long exposure to a high concentration of amyl acetate there is a 

 considerable time required for the residue of odorant to fall below a detect- 

 able level. 



A strong flow rate dependence of the rabbit olfactory response to amyl 

 acetate is shown by the records in Fig. 17. There is no evidence of plateau- 



ALL FIVE *T 

 -2 



10 



FLOW RATE 



ALL FOUR AT 



10 



FREE ReSPlHATION 



AMYL ACETATE AT MARKERS — CONCENTRATIONS AS EXPONENTIAL FRACTIONS OF SATURATION AT 20* C 



Fig. 17. Flow rate dependence of rabbit olfactory response. 



ing of the response as was typical for the tortoise. But it is difficult to 

 flow air through the passive nose of the rabbit at rates approaching the 

 higher values we believe are often attained momentarily during free respira- 

 tion. Perhaps with introduction of odorous gas through a surgical window 

 the flow rate dependence can be reduced sufficiently to get data suitable for 

 curve fitting. 



